The present invention generally relates to apparatus and methods for determining absolute values of various properties of a physiological medium. In particular, the present invention relates to non-invasive optical systems and methods for determining absolute values of concentrations of oxygenated and deoxygenated hemoglobins (and/or their ratios). The present invention also relates to apparatus and methods for obtaining such absolute values by solving generalized photon diffusion equations and their simplified variations such as modified Beer-Lambert equations.
Near-infrared spectroscopy has been used for non-invasive measurement of various physiological properties in animal and human subjects. The basic principle underlying the near-infrared spectroscopy is that physiological tissues include various highly-scattering chromophores to the near-infrared waves with relatively low absorption. Many substances in a physiological medium may interact or interfere with the near-infrared light waves that propagate therethrough. Human tissues and cells, e.g., include numerous chromophores such as oxygenated hemoglobins, deoxygenated hemoglobins, water, cytochromes, and lipids, where the hemoglobins are the dominant chromophores in the spectrum range of 700 nm to 900 nm. Accordingly, the near-infrared spectroscopy has been applied to measure oxygen levels in the medium such as tissue hemoglobin oxygen saturation (abbreviated as xe2x80x9coxygen saturationxe2x80x9d hereinafter) and total hemoglobin concentrations. Various techniques have been developed for the near-infrared spectroscopy, e.g., time-resolved spectroscopy (TRS), phase modulation spectroscopy (PMS), and continuous wave spectroscopy (CWS).
The TRS technology is based on operational principles such as pulse-time measurements and pulse-code modulation. In particular, it measures a time delay between an entry and an exit of electromagnetic waves to and from the physiological medium. Typically, the TRS applies to the medium an impulse or pulse sequence of electromagnetic waves having a duration in the order of a few pico-seconds. Photon diffusion encodes tissue characteristics not only in the timing of the delayed pulse received by a detector, but also in the received intensity time profile. Therefore, instead of receiving a xe2x80x9ccleanxe2x80x9d replica of the transmitted pulse, the return signals are spread out in time, and have greatly reduced amplitudes. Accordingly, the TRS measures the intensity of the return signals over a finite period of time, which is long enough to detect an entire portion of the delayed return signals. Based on such shape changes and amplitude attenuation of the input impulse or pulse, different times of arrival of photons and the mean time delay between the light (or wave) source and detector are used to obtain the tissue absorption and tissue scattering through, e.g., deconvolution of the return signals. Information on the tissues traversed (such as optical pathlengths and their changes) is then readily obtained. Details of the TRS technology are provided, for example, in D. A. Boas et al., Proc. Natl. Acad. Sci., vol. 91, p. 4887 (1994); R. P. Spencer and G. Weber, Ann. (N.Y.) Acad. Sci., vol. 158, p. 3631 (1996); and J. Sipior et al., Rev. Sci. Instrum., vol. 68, p. 2666 (1997), all of which are incorporated herein by reference for background.
The PMS technology employs phase-modulated electromagnetic waves irradiated by the wave source and transmitted through the physiological medium. Typical examples of PMS include homodyne systems, heterodyne systems, single side-band systems, and other systems based on transmitter-receiver cross-coupling and phase correction algorithms. Like TRS, PMS systems monitor the intensities of the attenuated electromagnetic waves. In addition, it is necessary for the PMS system to measure frequency-domain parameters, such as phase shift of the electromagnetic waves which is independent of the wave intensities. Based on such time-domain and frequency-domain information, PMS systems determine spectra of an absorption coefficient and/or scattering coefficient of the chromophores of the medium, and calculate absolute values of the hemoglobin concentrations. Details of the PMS technology are provided, for example, in U.S. Pat. No. 5,820,558 and a technical article by B. Chance et al. in Rev. Sci. Instrum., vol. 69, p. 3457 (1998), both of which are incorporated herein by reference in their entirety.
By contrast, CWS systems employ electromagnetic waves that are non-impulsive and not phase modulated. That is, CWS systems apply to the medium electromagnetic waves having at least substantially identical amplitude over a measurable period of time. On the detection side, CWS systems only measure intensities of the irradiated and detected electromagnetic waves and does not assess any frequency-domain parameters thereof.
In a homogeneous and semi-infinite medium model, both of the TRS and PMS have been used to obtain spectra of an absorption coefficient and (reduced) scattering coefficient of the physiological medium by solving a photon diffusion equation, and to calculate the absolute values of the concentrations of oxygenated and deoxygenated hemoglobins as well as tissue oxygen saturation. Despite their capability of providing such absolute values of the hemoglobin concentrations and the oxygen saturation, one major drawback of TRS and PMS is that the TRS equipment requires a pulse generator and detector and that the PMS needs additional hardware and signal processing capabilities to determine frequency-domain parameters. Accordingly, in practice both TRS and PMS systems are bulky and expensive. To the contrary, the CWS may be manufactured at a lower cost because all it needs to do is to perform intensity measurement. However, prior art CWS technology was limited in its utility because it can only provide the relative values or changes in the oxygenated and deoxygenated hemoglobin concentrations. Therefore, the conventional CWS cannot estimate the tissue oxygen saturation from such changes in the hemoglobin concentrations. Thus, there is a need for novel CWS systems and methods for measuring absolute value of concentrations of the hemoglobins and the oxygen saturation in the physiological medium.
The present invention generally relates to apparatus and methods for obtaining the absolute values of concentrations of chromophores of a medium and/or absolute values of their ratios. More particularly, the present invention relates to non-invasive optical systems and methods based on continuous wave spectroscopy (CWS) methods for determining absolute values of concentrations of the oxygenated and deoxygenated hemoglobins and their ratios in a physiological medium.
In general, wave propagation or photon migration in a medium is described by a generalized photon diffusion equation:                     I        =                  α          ·          β          ·          γ          ·                      I            o                    ·                      e                          {                                                                    -                    B                                    ·                  L                  ·                  δ                  ·                                                            ∑                      i                                        ⁢                                          (                                                                        ϵ                          i                                                ⁢                                                  C                          i                                                                    )                                                                      +                σ                            }                                                          (        1        )            
where xe2x80x9cI0xe2x80x9d is a system variable representing an intensity of the electromagnetic waves or photons (e.g., in the near-infrared ranges) irradiated by a wave source and where xe2x80x9cIxe2x80x9d is another variable denoting an intensity of the electromagnetic waves detected by a wave detector. Parameter xe2x80x9cxcex1xe2x80x9d is generally associated with the wave source and medium and accounts for, e.g., characteristics of the wave source such as irradiation power and configuration thereof, mode of optical coupling between the wave source and medium, and/or optical coupling loss therebetween. Parameter xe2x80x9cxcex2xe2x80x9d is generally associated with the wave detector and medium and accounts for, e.g., characteristics of the wave detector such as detection range and sensitivity, optical coupling mode between the wave detector and medium, and associated coupling loss. Parameters xe2x80x9cxcex1xe2x80x9d and xe2x80x9cxcex2xe2x80x9d may also depend upon, to some extent, other system characteristics and/or optical properties of the medium, including those of chromophores included therein. Parameter xe2x80x9cxcex3xe2x80x9d may be either a proportionality constant (including, e.g., 1.0) or a system parameter which may change its value according to the characteristics of the wave source, wave detector, chromophores, and/or medium. Parameter xe2x80x9cBxe2x80x9d generally accounts for lengths of optical paths of photons or electromagnetic waves traversed through the medium, and is predominantly determined by the optical properties of the medium. However, an exact value of parameter xe2x80x9cBxe2x80x9d may also depend on the characteristics of the wave source and/or detector as well. A typical example of such parameter xe2x80x9cBxe2x80x9d is conventionally known as a path length factor. It is noted that the parameter xe2x80x9cBxe2x80x9d may also take the value of 1.0 where the generalized diffusion equation (1) is approximated to the Beer-Lambert equation. To the contrary, parameter xe2x80x9cLxe2x80x9d is generally geometry-dependent and accounts for a (linear) distance between a particular wave source and a matching wave detector operatively coupled to each other. Parameter xe2x80x9cxcex4xe2x80x9d may be either a proportionality constant (including, e.g., 1.0) or a system parameter that may depend on the characteristics wave source, wave detector, and/or medium. Parameter xe2x80x9cxcex5ixe2x80x9d accounts for an optical interaction and/or interference of photons or electromagnetic waves with an i-th chromophore included in the medium. It is noted that, depending upon the definition and value of the parameter xe2x80x9cxcex4xe2x80x9d, the parameter xe2x80x9cxcex5ixe2x80x9d may represent an extinction coefficient, an absorption coefficient, a scattering coefficient, and/or a reduced scattering coefficient of the medium or the chromophores included therein. Variable xe2x80x9cCixe2x80x9d represents concentration of the i-th chromophore included in the medium, and parameter xe2x80x9c"sgr"xe2x80x9d is either a proportionality constant (including, e.g., 0.0) or a parameter which may be associated with the wave source, wave detector, and/or medium.
It will be appreciated that while the system variables such as xe2x80x9cIxe2x80x9d and xe2x80x9cI0xe2x80x9d are functions of time only, other system parameters have constant values which are determined by the characteristics of the wave source, wave detector, physiological medium, and chromophores included therein. Therefore, the generalized diffusion equation (1) is a function of time and is independent of or at least substantially insensitive to frequency-domain parameters, such as the frequency and phase angle of the electromagnetic waves. Despite the numerous parameters of the generalized photon diffusion equation (1) and various modified versions thereof as will be described in greater detail below, the optical systems and methods disclosed in this invention enable the direct determination of absolute values of the chromophore concentrations and/or ratios thereof.
In one aspect of the present invention, a method is provided to solve a set of wave equations applied to an optical system having at least one wave source and at least one wave detector. Photons or electromagnetic waves are irradiated by the wave source, transmitted through the physiological medium including at least one chromophore, and detected by the wave detector. The wave equation, e.g., the generalized diffusion equation (1), expresses the intensity of electromagnetic waves detected by the wave detector (i.e., xe2x80x9cIxe2x80x9d) as a function of system variables (e.g., xe2x80x9cI0xe2x80x9d and xe2x80x9cCixe2x80x9d) and system parameters (e.g., xe2x80x9cxcex1,xe2x80x9d xe2x80x9cxcex2,xe2x80x9d xe2x80x9cxcex3,xe2x80x9d xe2x80x9cB,xe2x80x9d xe2x80x9cL,xe2x80x9d xe2x80x9cxcex4,xe2x80x9d xe2x80x9cxcex5i,xe2x80x9d and xe2x80x9c"sgr"xe2x80x9d). The method generally includes the steps of obtaining one or more sets of equations by applying the wave equation to the optical system capable of irradiating multiple sets of electromagnetic waves having different wave characteristics, eliminating the source-dependent parameter (e.g., xe2x80x9cxcex1xe2x80x9d) and detector-dependent parameter (e.g., xe2x80x9cxcex2xe2x80x9d) therefrom to obtain a set of intermediate equations, providing at least one correlation of chromophore concentrations (and/or their ratios) with medium-dependent and geometry-dependent parameters (e.g., xe2x80x9cBxe2x80x9d and xe2x80x9cL,xe2x80x9d respectively), incorporating the correlation into the set of intermediate equations, and obtaining an expression for absolute values of the concentrations of the chromophores (and/or ratios thereof) in terms of the correlation, intensities of electromagnetic waves (e.g., xe2x80x9cIxe2x80x9d and xe2x80x9cI0xe2x80x9d), and medium- or chromophore-dependent parameters (e.g., xe2x80x9cxcex5ixe2x80x9d). The method also includes the steps of determining a value of the correlation from a known geometric arrangement between the wave source(s) and detector(s), experimentally measuring the intensities of the electromagnetic waves (i.e., xe2x80x9cIxe2x80x9d andxe2x80x9cI0xe2x80x9d) determining values of medium- or chromophore dependent parameters (i.e., xe2x80x9cxcex5i""sxe2x80x9d), and determining the absolute values of the concentrations of the chromophores (and/or ratios thereof) therefrom.
This embodiment of the present invention offers several benefits over the prior art CWS technology, which is capable of measuring only the changes (i.e., differential or relative values) in the chromophore concentrations. In particular, the foregoing as well as the following embodiments of the present invention allow determination of such values and/or ratios only by measuring the intensities of the electromagnetic waves irradiated by the wave source and detected by the wave detector. Accordingly, the present invention does not require bulky, complex, and/or expensive electronic parts to measure temporally perturbed pulse signals of a short duration (as used in TRS) and to assess frequency-domain parameters of the electromagnetic waves (as used in PMS). For example, the embodiments of the present invention provide direct means for assessing the xe2x80x9cabsolute valuesxe2x80x9d of the chromophore concentrations as well as their ratios in various physiological media, e.g., tissues or cells in organs, muscles, and/or body fluids. In addition, as will be described in detail below, the foregoing method of the present invention can be readily applied into optical probes having any arbitrary number of wave sources and/or detectors arranged in any arbitrary configuration. Furthermore, the foregoing embodiment of the present invention cal also be applied to any optical probes of conventional optical imaging systems including any number of wave sources and detectors arranged in any arbitrary configurations. Thus, the foregoing method allows construction of optical systems customized to specific clinical applications without compromising their performance characteristics.
Embodiments of this aspect of the present invention may include one or more of the following features.
The generalized diffusion equation (1) may be applied to an optical system with at least one wave source and at least one wave detector:                               I          mn                =                              α            m                    ·                      β            n                    ·          γ          ·                      I                          o              ,              m                                ·                      e                          {                                                                    -                                          B                      mn                                                        ·                                      L                    mn                                    ·                  δ                  ·                                                            ∑                      i                                        ⁢                                          (                                                                        ϵ                          i                                                ⁢                                                  C                          i                                                                    )                                                                      +                σ                            }                                                          (        2        )            
where the subscripts xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d represent an m-th wave source and an n-th wave detector, respectively.
The method may include the steps of applying equation (2) to the optical system to obtain a first and a second set of equations, eliminating at least one of xcex1m, xcex2n, xcex3, xcex4, and "sgr" from the first and second set of equations by performing mathematical operations thereon to obtain a third set of equations, providing at least one theoretically-derived, semi-empirical or empirical correlation between the concentrations of the chromophores (or ratios thereof) and one or more terms of the third set of equations including Bmn and/or Lmn, incorporating the above correlation into the third set of equations to replace such terms thereby, and obtaining an expression for the absolute values of the concentrations of the chromophores (and/or ratios thereof) based on the measured values of Imn and Io,m and known values of xcex5i""s.
The foregoing method may also include the steps of applying the optical system to the physiological medium such as cells of organs, tissues, and body fluids, and determining the absolute values of the chromophore concentrations (and/or their ratios) directly from the experimentally measured values of Imn and Io,m and known values of xcex5i""s. The measuring step may include an additional step of monitoring concentrations of oxy-hemoglobin and/or deoxy-hemoglobin, and/or a ratio thereof such as, e.g., (tissue) oxygen saturation.
The foregoing method may also include the step of determining presence of tumor cells in a finite area of the medium or determining a presence of an ischemic condition as well. In the alternative, the foregoing method may also include the steps of applying the foregoing optical system to the physiological medium including transplanted cells of organs and/or tissues and measuring absolute values of the chromophore concentrations (or their ratios) based on the measured values of Imn and Io,m and known values of xcex5i""s. For example, the method may be used to determine presence of an ischemic condition in the transplanted organs and tissues during or after surgical procedures.
The applying step of the foregoing method may include the step of irradiating the first and second set of electromagnetic waves which can be preferably distinguished by the wave detector due to their different wave characteristics. For example, different sets of the electromagnetic waves may have different wavelengths (or frequencies), phase angles, harmonics, and/or a combination thereof. Therefore, in the irradiating step, the first set of electromagnetic waves are irradiated at a first wavelength, while the second set of electromagnetic waves has a second wavelength that is different from the first wavelength.
The eliminating step of the foregoing method may include the step of deriving at least one first ratio of two wave equations both of which are selected from one of the first and second sets of the equations. The wave equations may be applied to the same wave source but to different wave detectors, thereby eliminating xcex1n, xcex3, and "sgr" from the first ratio. Alternatively, the wave equations may be applied to two different wave sources but to the same wave detector, thereby eliminating xcex2n, xcex3, and "sgr" from the first ratio. The eliminating step may also include the step of deriving at least one second ratio of two wave equations both of which are selected from the other of the first and second sets of the equations. A sum of or a difference between the first and second ratios may also be obtained so as to eliminate at least one of xcex1m and xcex2n. In the alternative, the eliminating step may include the step of approximating both parameters xe2x80x9cxcex3xe2x80x9d and xe2x80x9cxcex4xe2x80x9d as a unity.
The providing step of the foregoing method may include the step of providing a formula of the medium-dependent and geometry-dependent parameters as a polynomial, sinusoid or other functions of the chromophore concentrations (and/or ratios thereof). Such a formula may also include a zero-th order term. Alternatively, the medium-dependent and geometry-dependent parameters may be approximated as a constant.
In another aspect of the present invention, an optical system is provided to determine absolute values of concentrations of chromophores and/or ratios thereof in a physiological medium. The optical system typically includes a source module, a detector module, and a processing module. The source module irradiates, into the medium, two or more sets of photons or electromagnetic radiation having different wave characteristics. The detector module detects the electromagnetic waves which are transmitted through the medium. The processing module operatively couples with the detector module and determines an absolute value of the concentration of the chromophores and/or ratios thereof from electromagnetic radiation irradiated from and detected by the source and detector modules, respectively, where such determination is based only on the intensity measurement of continuous wave electromagnetic radiation. It is noted that the source and detector modules are preferably designed to operate in a continuous-wave spectroscopy (CWS) mode.
This embodiment of the present invention offers several benefits over the prior art near-infrared spectroscopy technologies such as conventional CWS, TRS, and PMS. As discussed above, the prior art CWS can measure only relative values of or changes in the concentration of the chromophores. Therefore, the conventional CWS can neither provide the absolute values of the chromophore concentrations nor the ratios thereof such as (tissue) oxygen concentration. By contrast, the optical monitoring or imaging systems based on the present invention operating in the CWS mode can measure the absolute values of the chromophore concentrations (and their ratios) and provide images of two-dimensional or three-dimensional distribution of such absolute values. In addition, because the optical systems of the present invention only require intensity measurement of the electromagnetic radiation (Imn and Io,m) without having to process the frequency-domain characteristics, they can be provided as cheap, portable, but reliable devices. In addition, when the foregoing systems of the present invention may be readily modified and applied to the conventional TRS and PMS technologies for improved accuracy and lower cost.
Embodiments of this aspect of the present invention may include one or more of the following features.
The wave source may be arranged to irradiate electromagnetic waves which can be preferably distinguished by the wave detector due to their different wave characteristics. For example, different sets of the foregoing electromagnetic waves may have different wavelengths (or frequencies), phase angles, harmonics or their combination. For example, the first set of the electromagnetic waves may have a first wavelength and a second set of said electromagnetic waves may have a second wavelength which is different from the first wavelength. In the alternative, the first set of electromagnetic waves may be carried by a first carrier wave and the second set of electromagnetic waves may be carried by a second carrier wave which has wave characteristics different from those of the first carrier wave, e.g., different wavelengths, phase angles, harmonics, etc. It is appreciated that different wave characteristics of the electromagnetic waves are necessary for the wave detector only to obtain multiple intensity signals (i.e., Imn and Io,m measured at different wavelengths) over the same sampling area of the medium. However, such different wave characteristics are not directly used to determine the absolute values of the chromophore concentration and/or their ratios.
The processing module preferably includes an algorithm to determine the absolute values of the chromophore concentrations (or their ratios) based on various system variables and/or parameters, e.g., the intensity of the electromagnetic waves irradiated by the source module, intensity of the electromagnetic waves detected by the detector module, and one or more system parameters accounting for interaction or interference of electromagnetic waves and/or photons with the medium.
The wave equations may include at least one term which is substantially dependent on the optical properties of the medium (i.e., medium-dependent) and/or on configuration of the source and detector modules (i.e., geometry-dependent). Examples of such term may include, but not limited to, xe2x80x9cBxe2x80x9d and xe2x80x9cLxe2x80x9d of equation (1) or xe2x80x9cBmnxe2x80x9d and xe2x80x9cLmnxe2x80x9d of equation (2). The algorithm of the processing module may include at least one correlation expressing a first function of the term as a second function of the chromophore concentrations (and/or ratios thereof). The second function may be any analytic function, e.g., a polynomial of the concentrations and/or ratios thereof. Alternatively, the algorithm may also be arranged to approximate the second function as a constant.
The source module may include at least one wave source, and the detector module may include at least two wave detectors. Alternatively, the source module may include at least two wave sources, while the detector module may include at least one wave detector. It is preferred, however, that both of the source and detector modules include, respectively, at least two wave sources and at least two wave detectors.
In one aspect of medical application of the present invention, the foregoing optical systems and methods therefor may be used to measure the absolute values of concentrations of oxygenated and deoxygenated hemoglobin and/or their ratio. Such optical systems will be beneficial in non-invasively diagnosing ischemic conditions and/or locating ischemia in various organs and tissues. For example, the optical system may be used to prognose or diagnose stroke, cardiac ischemia or other physiological abnormalities originating from or characterized by abnormally low concentration of oxy-hemoglobin. Accordingly, presence of cancerous tumors may be easily detected. In addition, the optical systems and methods of the present invention may be applied to tissues or cells disposed in epidermis, corium, and organs such as a lung, liver, and kidney.
In another aspect of the medical application of the present invention, the foregoing optical systems and methods therefor may be applied to measure the absolute values of the concentrations of oxy- as well as deoxy-hemoglobins to diagnose vascular occlusion during or after various surgical procedures including organ transplantation. In general, prognosis of organ transplantation depends on adequate supply of oxygenated blood to transplanted organs during and post surgical procedure. The optical systems and methods of the present invention may be applied to detect vascular occlusion in transplanted heart, lung, liver, and kidney in its earliest stage.
In a further aspect of the medical application of the present invention, the foregoing optical systems and methods may be applied to assess absolute properties of the substances included in the physiological medium. Examples of such substances may include, but not limited to, concentrations (or their ratios) of blood, lipids, cytochromes, water, and/or other chromophores in the medium.
The foregoing systems and methods of the present invention may be employed for various applications, e.g., non-invasively disposed on the medium or, alternatively, to be invasively disposed on an internal medium.
As used herein, a xe2x80x9chemoglobinxe2x80x9d or xe2x80x9chemoglobinsxe2x80x9d refer to oxygenated hemoglobin and/or deoxygenated hemoglobin. In addition, the xe2x80x9chemoglobin,xe2x80x9d xe2x80x9chemoglobins,xe2x80x9d and/or xe2x80x9cvalues of hemoglobinsxe2x80x9d represent properties of such xe2x80x9chemoglobins.xe2x80x9d Examples of such properties may include, but not limited to, amount or concentration thereof, total amount or total concentration thereof which corresponds to the sum of each amount or concentration of the oxygenated and deoxygenated hemoglobins, respectively.
A xe2x80x9cchromophorexe2x80x9d refers to any substance in a physiological medium optically interacting with photons or electromagnetic waves transmitting therethrough. Chromophore generally includes solvents of a physiological medium, solutes dissolved in such a medium, and/or other substances included in the medium. Specific examples of such chromophores may include, without limitation, cytochromes, enzymes, hormones, neurotransmitters, chemo- or chemical transmitters, proteins, cholesterols, apoproteins, lipids, carbohydrates, cytosols, cytosomes, blood cells, water, hemoglobins, and other optical materials present in animal or human cells, tissues or body fluid. Chromophores also include extra-cellular substances which may be injected into the medium for therapeutic and/or imaging purposes and may interact with electromagnetic waves. Such chromophores may include, without limitation, dyes, contrast agents, and other image-enhancing agents, each of which may exhibit optical interaction with electromagnetic waves having wavelengths in a specific range.
xe2x80x9cElectromagnetic wavesxe2x80x9d as used herein generally refer to acoustic or sound waves, near-infrared rays, infrared rays, visible light rays, ultraviolet rays, lasers, and/or rays of photons.
xe2x80x9cPropertyxe2x80x9d of the chromophores may mean intensive or extensive property thereof. Examples of such intensive property may include, but not limited to concentration of the chromophore, a sum of such concentrations, and a ratio thereof. Examples of extensive property may include, without limitation, volume, mass, weight, volumetric flow rate, and mass flow rate of the chromophores.
The term xe2x80x9cvaluexe2x80x9d is an absolute value of the chromophore property. The term xe2x80x9cvaluexe2x80x9d may also refer to a relative value representing spatial or temporal changes in the property of the chromophores including deoxygenated and oxygenated hemoglobins.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood and/or used by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be applied and/or used in the practice of or testing the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present application, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.